An ultrapure water producing system generally comprises a pretreatment system, a primary water purifying system, and a subsystem. The pretreatment system comprises a clarification apparatus using such as a coagulation filter, an MF membrane (microfiltration membrane), or a UF membrane (ultrafiltration membrane) and a dechlorination apparatus using such as activated carbon.
The primary water purifying system comprises an RO (reverse osmosis membrane) apparatus, a deaerating membrane apparatus, and an electrodeionization apparatus, thereby removing-most of ion and TOC (Total Oxidizable Carbon).
The subsystem comprises an UV apparatus (ultraviolet oxidation apparatus), a nonregenerative-type ion-exchange apparatus, and an UF apparatus (ultrafiltration apparatus), thereby removing trace of ion, particularly removing trace of organic matter of low molecular, and removing microparticles.
Ultrapure water produced in the subsystem is sent to a use point and surplus ultrapure water is commonly returned to a tank of a former stage of the subsystem.
The required quality of ultrapure water has been increased every year. In most advanced electronic industrial field, ultrapure water having boron concentration of 10 ppt or less is presently required.
To produce ultrapure water having such low boron concentration, JP H08-89956A describes that raw water is desalted in a desalter such as an RO membrane apparatus and is then fed through a boron adsorptive resin tower.
JP H09-192661A describes in a paragraph 0040 that as shown in FIG. 3, raw water is treated by feeding the raw water through an RO membrane apparatus 31, an electrodeionization apparatus 32, and a boron adsorptive resin tower 33, in this order.
Electrodeionization apparatuses capable of removing silica and boron are described in JP 2001-113281A (U.S. Pat. No. 6,379,518B1) and JP 2002-205069A (U.S. Pat. No. 6,733,646B2).
When raw water is fed through the RO membrane apparatus 31, the electrodeionization apparatus 32, and the boron absorptive resin tower 33 as shown in FIG. 3, boron is removed both in the electrodeionization apparatus 32 and the boron absorptive resin tower 33, thereby producing treated water having low boron concentration. When the boron absorptive resin tower 33 is arranged in a subsequent stage of the electrodeionization apparatus 32, however, a need of regenerating or exchanging the resin in the boron absorptive resin tower arises at a breakthrough point P1 where a small amount of boron starts to leak as shown in FIG. 2, in order to adjust the boron concentration level in the water flowing out of the boron absorptive resin tower to satisfy a target water quality. That is, the water purifying system of JP H09-192661 has a problem of low production efficiency of pure water because of increased frequency of regenerating or exchanging the boron absorptive resin.
To increase the water recovery rate of the electrodeionization apparatus, it can be considered to return concentrated water of the electrodeionization apparatus to the upstream side of the RO membrane apparatus so that the concentrated water is added into raw water. In this case, water containing boron is circulated between the electrodeionization apparatus and the RO membrane apparatus so that the boron is concentrated therebetween. That is, the boron removing rate of the RO membrane apparatus is about 50% and about half of the boron in the concentrated water from the electrodeionization apparatus passes through the RO membrane apparatus and then enters into the electrodeionization apparatus. As a result of repeating, this process, the boron concentration in water flowing from the RO membrane apparatus to the electrodeionization apparatus is increased. As a result, the boron load on the boron absorptive resin apparatus in a subsequent stage of the electrodeionization apparatus is increased.